1. Field of the Invention
This invention relates to an electric motor used, for example, for driving a seat of a power seat apparatus of an automotive vehicle and, more particularly to a motor with a built-in rotation detecting mechanism for detecting the position of the seat correlatively according to the number of rotations of an output shaft of the electric motor.
2. Description of the Prior Art
There has been known an electric motor shown in FIG. 4 as an electric motor housed with a rotation detecting mechanism for driving the seat and detecting the present position of the seat correlatively according to the rotation of the motor in the power seat apparatus of the automotive vehicle.
An illustrated electric motor 50 is provided with an armature 52 in a motor case 51, a worm 53a formed on a shaft 53 of the armature 52 is meshed with a worm wheel 54, and the worm wheel 54 is secured through a cross-shaped plate 62 to an output shaft 57 supported rotatably by a bearing 56 disposed in a gear case 55. The output shaft 57 is secured with an output gear 58 on the outside of the gear case 55.
According to electric supply to the armature 52, the worm wheel 54 rotates by the rotary power of the armature 52 and the output gear 58 rotates by the rotation of the worm wheel 54 through the cross-shaped plate 62 and the output shaft 57, thereby tilting forward or backward the seat back connected to the output shaft 57 through the output gear 58.
The output shaft 57 is secured with a wheel-shaped magnet 59 which is polarized with south and north poles alternately on the outer periphery thereof, and a reed switch 60 is attached on a gear case cover 61 covering the opening side of the gear case 55 so as to be sited at a predetermined distance away from the outer periphery of the magnet 59.
When the output shaft 57 rotates according to the rotation of the armature 52, the magnet 59 rotates together with the output shaft 57, whereby the reed switch 60 disposed on the outer side of the magnet 59 becomes on then off alternatingly according to alteration of the magnetic force caused by the rotation of the magnet 59. And a controller (not shown) detects the position of the seat or the tilt angle of the seat back by counting the number of pulses generated from the reed switch 60 repeating on and off states alternatingly.
However, since the magnet 59 is secured to the output shaft 57 in the conventional electric motor 50 as mentioned above, if there is a play in the magnet 59 or the output shaft 57 in the rotational direction due to a gap between the magnet and the output shaft 57, a backlash between the worm wheel 54 and the worm 53a of the armature shaft 53, or a play of the armature shaft 53 in the longtitudinal direction, there is the possibility that the pulse signal is generated from the reed switch 60 in response to the small turn of the magnet 59 caused by the play of the magnet 59 or the output shaft 57 even when the output shaft 57 does not rotate. In such a case, there is a problem in that error appears between the actual seat position and the seat position obtained by detecting the rotation of the output shaft 57 of the electric motor 50 through the reed switch 60 and the magnet 59.